SIEMENS

A second key innovation is the IRIS method (Iterative Reconstruction in Image Space), which involves sophisticated image processing software. Its job is to detect structures in a noisy image and sharpen them. A CT image is made up of many hundreds of individual images taken at different angles as the measuring system circles the patient. Normally the final image is computed all at once from these individual images. However, for physical reasons each recorded value is subject to a certain inaccuracy that is visible in the form of noise, which is a bit like the static on the TV picture you get when reception is poor.

The IRIS method helps to produce a more detailed image by post-correcting the data in a multistep process. It identifies details such as blood vessels and the bone edges in an initial image, sharpens these, and reduces the noise in the remaining parts of the image. This process is repeated three to five times until the details have been reliably resolved. The entire IRIS analysis is completed in just fractions of a second. As the method enables more image information to be extracted from a scan, the image can be recorded with less radiation. “The dose can be lowered by as much as 60 % compared to scans without IRIS,” says Thomas Flohr, Head of the CT Physics department in Forchheim. “And that's a big reduction.”

The third technology is known as Adaptive Dose Shield. Here, X rays are focused on the target with an accuracy of a thousandth of a millimeter, thus helping to cut dose. Anyone who has ever tried to trace the outline of an object with a flashlight knows how much light spills over the edges with a wide beam. It is a similar situation with X rays — the surrounding tissue is exposed unnecessarily to radiation. In the Definition Flash, however, X rays are kept from dispersing in the same way that a horse's field of view is restricted by blinders. As a result, the dose is reduced by between ten and 35 %, depending on whether the entire chest or just the heart is being scanned.

The combination of all of these methods results in an astonishing overall reduction. “Just a few years ago, the dose for a cardiac examination was ten to 30 mSv,” says Flohr. “Our goal is to get the radiation exposure down to less than one millisievert for more than just heart scans. We want to significantly reduce it for all types of examinations in the medium term.” The X ray dose for an abdominal examination is currently about 8 to 10 mSv. Flohr hopes to reach 2 to 3 mSv in the near future. “I even think it will be possible in the near future to perform all routine examinations with a dose below the natural annual background dose.”

Measuring Individual Photons. Siemens is already working on the development of additional dose-reducing technologies. A photon-counting detector, for instance, which converts X ray radiation into image information directly and efficiently is an example. After passing through the body, X ray quanta currently first strike a ceramic layer that converts the energy into pulses of light. These are detected by a photodetector and counted. The problem is that the light quanta can zoom off in different directions, interfere with adjacent photodetectors, or be lost entirely, thus blurring the image. Another disadvantage of the ceramic is that because the material luminesces briefly, it is difficult to differentiate between individual quantum events — and the detector takes an average of several incident quanta arriving one after the other.

In the photon-counting detector, a prototype of which is currently undergoing testing, X ray quanta strike a semiconducting layer, where they trigger an electrical voltage pulse that is measured directly. The advantage of this method is that it allows the energy of each individual X ray quanta to be measured, which increases the efficiency of the detector and significantly reduces radiation dose. “Furthermore, we can now detect X ray quanta with different energies,” says Flohr.

This opens the door for the first time to using computed tomography to deduce the nature of the substance scanned. It is sometimes barely possible to differentiate between bones and blood vessels filled with a contrast agent in a CT image. But thanks to precise quantum analysis, tomorrow's medical personnel may be able to tell what substance the radiation actually scanned.

Low-radiation CT will also help to reduce costs. Today, patients who exhibit equivocal symptoms of heart disease are often examined using an expensive, catheter-based interventional procedure in which the position of the catheter in the body has to be monitored using X rays. A CT scan could be an alternative. “In roughly 60 % of all catheterizations, it turns out that there is no serious pathology,” says Peter Seitz, head of CT Product Marketing at Siemens. All told, a CT exam takes just a few minutes and is vastly less expensive than catheterization, despite the fact that it is based on the use of scanner that can cost up to 2 mill. €.